Thick dielectric electroluminescent (TDEL) displays provide a great advance in flat panel display technology. TDEL displays comprise a basic structure including a ceramic or other heat resistant substrate upon which an electrically conductive film is deposited to define a set of row electrodes. A thick film dielectric layer consisting of a ferroelectric material overlies the row electrodes and is sandwiched between the electrically conductive film and a thin film structure. The thin film structure includes one or more thin film dielectric layers sandwiching one or more phosphor films. A set of optically transparent column electrodes is deposited on the thin film structure. The entire basic structure is covered with a sealing layer to protect the thick and thin film dielectric layers from degradation due to moisture and/or other atmospheric contaminants.
The composite dielectric layers used in these TDEL displays have a high dielectric constant, allowing the use of relatively thick dielectric layers in the TDEL displays without a significant increase in the display operating voltage. However, the dielectric breakdown strength of these dielectric materials is relatively low, necessitating the use of a relatively thick dielectric layer, typically greater than 10 micrometers, to prevent dielectric breakdown during display operation. Typically, the thick film dielectric layer comprises a sintered perovskite piezoelectric or ferroelectric material such as lead magnesium niobate (PMN) or lead magnesium niobate-titanate (PMN-PT) with a dielectric constant of several thousand. There may also be a thinner overlayer of a compatible plezoelectric or ferroelectric material such as lead zirconate titanate (PZT) applied using metal organic deposition (MOD) or sol gel techniques to smooth the surface of the thick film dielectric layer in preparation for deposition of the thin film structure.
Typically, the thick film dielectric layer is sintered at about 850° C. onto an alumina substrate onto which thin film gold row electrodes have been deposited. This yields a sintered thick film dielectric layer having a density that is sufficiently high that the remaining porosity, particularly in the upper portion of the thick film dielectric layer can be filled using sol gel or MOD techniques. If the thick film dielectric layer is sintered at temperatures lower than 850° C., the residual porosity of the thick film dielectric layer after firing is increased. This makes it more difficult to smooth the surface of the thick film dielectric layer using a sol gel or MOD process, and tends to reduce the dielectric constant and dielectric strength of the composite dielectric layer.
Using glass material (i.e. glass or glass ceramic) substrates that are commercially available in large areas is of course desired in order to reduce TDEL manufacturing costs. However, if such a glass material substrate is used, the softening temperature of the glass material substrate must not be exceeded during the sintering process in order to avoid warping of the glass material substrate. Since the softening temperature of glass material substrates is near 650° C., sintering must be carried out at temperatures below 650° C. but by doing so, the above-described problems are encountered. As a result, known thick film dielectric layers remain incompatible with glass material substrates.
It is therefore an object of the present invention to provide a novel low firing temperature thick film dielectric layer for an electroluminescent display and to an electroluminescent display incorporating the same.